Castings containing voids cannot be used in many applications. Besides having a weaker structure than comparative solid castings, castings having voids can become softer and yield from the pressure in the voids when their temperature is raised. Further, blisters can form and rupture, thereby causing problems to component performance.
Typically, there are three different types of voids or porosities that occur in metal castings; voids caused by the metal not reaching portions within the mold, voids formed by trapped gas or gas formation and voids caused by shrinkage of the metal when it solidifies. In order to remove the voids from a metal casting, it is known in the prior art to first cast a component in a casting apparatus. After solidification, the cast part is then removed from the casting apparatus and mold and placed in a hot isostatic press (HIP) apparatus. Essentially, the HIP apparatus reheats the casting to the working portion of its solid phase while increasing to very high pressures (30,000-60,000 psi), thus, densifying the casting by softening the metal and mechanically forcing the voids closed.
This two-stage process of formation and densification has many problems though. The secondary HIP process is very time consuming, expensive and not always successful. The HIP process normally takes from 4-48 hours and requires pressures of 30,000-60,000 psi. HIP equipment for aircraft size components cost millions of dollars and is expensive to set up and operate. Furthermore, in the HIP process, the component must be maintained below its melting temperature. If the temperature goes too high, the component will deform since it is no longer in contact with the mold that was used to establish its shape.
FIGS. 1a-1e show a casting process which is known in the prior art. Typically, as shown in FIG. 1a, molten metal is introduced into a mold. Trapped gases and unfilled voids often exist at this stage. As shown in FIG. 1b, the molten metal cools through its liquid phase. Many molten metals chemically react with the mold materials, which can cause additional gas formation in the molten metal. Reactions which cause gas can occur with both monolithic and composite systems at different phases during the casting. In monolithic casting reactions can occur at the different phases of the metal at different temperatures. Reactions often occur with the mold material, the casting atmosphere, and the dissolved gases as they come out of solution. Pressure can also affect the amount of reaction--more gas molecules normally cause greater reactions, as does higher levels of contact with the mold.
The same problems occur and are more complex in the formation of composites. When reinforcements are infiltrated, it is not uncommon for the metal to react with either oxides on the reinforcement or the reinforcement itself. Dissolved gases may also react with the reinforcement in solution or as they come out of solution. These reactions can be either small or violent and are normally exothermic and create gases which can end up in the casting in the form of porosity. Next, as shown in FIG. 1c, the metal cools through its liquid/solid phase were shrinkage voids start to form and dissolved gas can come out of solution.
In the liquid/solid phase of aluminum, shrinkage voids are normally around 4% of the casting volume. This depends on the nature of this phase and decreases with alloys that are closer to eutectic. To solve the problem of liquid/solid phase shrinkage, a number of steps are normally taken as standard casting practice: metal is introduced at a higher temperature than the mold itself, extra metal is added to a larger cross section then actually needed for the shape in the mold with additional sprues and risers, and the sprues and risers are made very large to provide the extra hot metal. The goal is to have the sprues and risers be the last areas to solidify. These methods act to reduce the amount of shrinkage inside the part. However, accomplishing these steps successfully often requires significant trial and error on the sprue design, mold and melt temperatures. It is often difficult or impossible to remove all of this shrinkage from castings with standard casting practices.
Next, as shown in FIG. 1d, the metal is essentially solidified and cools through its solid phase. Solid phase shrinkage, for 6061 aluminum for example, is 0.00023 inches of material for every fahrenheit degree drop.
Consequently, as shown in FIG. 1e, the solidified metal parts contains various forms of voids which were formed at various stages throughout its cool down. In order to diminish or eliminate this porosity, the parts are removed from the mold and then placed on holders in a hot isostatic press (HIP) to force the voids closed. This secondary process is very time consuming, expensive and not always successful. It involves the parts being reheated to a temperature below the liquid/solid phase while pressure is increased to very high pressure to mechanically work the metal into the voids. The process normally takes from 4 to 48 hours and requires pressures of 30,000 to 60,000 psi. The equipment for HIPing aircraft size components costs millions of dollars and is expensive to set up and operate.
In HIPing, the part is heated to the working portion of its solid phase so that the part does not change shape. If the temperature of the metal is too high the metal will reflow and no longer be the desired part shape. The metal must be taken high enough in temperature in a high pressure atmosphere so that it softens and the high pressure on the surface of the part transfers through the part and mechanically works to crush any voids inside. The voids are not completely removed, just greatly reduced in size. Because the metal is not liquid it does not flow easily. Very high pressures on the order of 30,000-60,000 PSI are required. The thicker the part, the higher the pressure and the longer the time required to close the voids and the greater the difficulty in closing all the voids.
The present invention facilitates casting and densification in one operation. The method is significantly faster than previous known casting/densification processes and requires less expensive equipment and lower pressures for equivalent degrees of densification.